As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
Underwriters Laboratories (UL) 60950-1 Standard for Safety requires that all systems accepting power greater than allowed under Section 2.5 Limited Power Sources (LPS) shall have two steps of independent fault protection. These fault protection circuits add design complexity, board space, and cost. For a system to be consider LPS compliant and exempt from the overcurrent protection (OCP), output current cannot exceed 8 amperes for less than or equal to 30 volts output, and output apparent power cannot exceed 100 volt-amperes (VA). For example, maximum output power of a 90 Watt AC adapter having a 20 Volt output can potentially exceed 100 VA considering a worst case tolerance. In this example, output power needs to be under 100 VA=VO (no load). With the introduction USB Type C, maximum rated adapter power can be as high as 100 W.
In the traditional 90 Watt adapter design, it is not possible to meet the above mentioned LPS limits due to tight output tolerance requirement. FIG. 1A illustrates a diagram of primary side circuitry 190 of a conventional AC adapter that utilizes total power detection on the primary side to monitor the output power. Comparator 100 output goes high when the PWM driving signal is high, and this signal is inverted at the input to the NOR 103a causing its output to go high and turn ON the upper NPN transistor 104a of the push-pull circuit 104, which in turn turns ON MOSFET transistor Q1 105 and connects the primary coil 112 of the transformer to ground. A comparator 101 monitors the voltage drop across sense resistor Rsense 106. Comparator 101 trips if the voltage exceeds a predetermined threshold (Vth) and sets the latch 102. Latch 102 then turns ON the lower NPN transistor 104b of the push-pull circuit 104 which in turn overrides the PWM control and forces the gate voltage to transistor Q1 105 low, thus disabling the AC adapter. Although this simple circuit 190 works, its accuracy is poor due to high component tolerances.
Empirical data show tolerance of a conventional 90 Watt AC adapter exceeds 10 Watts and is therefore unable to meet the LPS safety limit of ≦100 VA. Existing 90 Watt adapters cannot meet LPS requirement due to electronic components tolerance deviation that results in a very large range on the OCP point. Thus output power of conventional AC adapters are typically undersized to cover electronic component tolerance deviation, e.g., such as downsizing a 90 Watt adapter design to be 85 Watts to gain more room to cover electronic tolerance range. However, such intentional adapter power undersizing to meet the 100 VA limit penalizes the adapter capability and impacts backward adapter compatibility.
Conventional LPS designs are also known that support single output voltage by using independent OCP circuits such as a primary side OP-Amp and current sensing resistor. However, such designs are not able to provide a very precise OCP point due to electronic components tolerance deviation and therefore cannot meet the LPS request. Type C adapters are typically design to support multiple output voltage levels (at same maximum power). Therefore, the OCP circuit must comprehend the negotiated power contract and set the corresponding trip point for that particular output voltage level.
As shown in FIG. 1B, it is known to use a secondary-side sense resistor placed in series in the ground return path from a power-consuming computer system load to a secondary-side transformer coil of an AC adapter. It is also known to provide an AC adapter with a secondary-side microcontroller (MCU) having an op amp comparator coupled to detect voltage drop the sense resistor while current is supplied from the AC adapter to the system load. In such a conventional configuration, a digital core of the MCU compares the measured voltage drop to a voltage threshold (Vth) that corresponds to the OCP point. If this measured voltage drop exceeds the Vth, then the MCU digital core shuts down the adapter by shutting down (or turning “OFF”) an output protection switch placed in the output power path from the secondary side of the AC adapter. As shown, an optocoupler is also present for purposes of detecting and providing a secondary side output voltage feedback to a primary-side PWM integrated circuit (IC) of the AC adapter.